You didn't miss it, Cammomile, it wasn't there initially, it's been added. Geoban, trimming roots is a professional job, and not one I would attempt other than on plants like ivy or vinca, which put up with quite a lot of abuse. I'm not even sure you can trim the roots of conifers - you would be much better making your selection of plants for a miniature garden from dwarf conifers and other small versions of plants - the appendix 'nana' in a plant's name means it's small, so I recommend you choose more appropriate plants.